Water-soluble cellulose ether is obtained by treating a cellulose having crystalline and amorphous moieties within the molecule with an etherifying reagent for converting the crystalline moieties to an amorphous state whereby the cellulose is made water soluble. It is acknowledged that the crystallinity of cellulose is largely ascribed to hydrogen bonds between hydroxyl groups within the molecule which develop on the skeleton structure of cellulose molecule (see Cellulose Society Ed., “Encyclopedia of Cellulose,” 2000, Asakura Publishing Co., Ltd.). Cellulose is water insoluble because the hydrogen bonds are strong enough to prevent hydration with water molecules in water. In the preparation of cellulose ether, cellulose is treated with an alkaline aqueous solution such as NaOH to convert to alkali cellulose for disrupting crystallinity, then reacted with an etherifying reagent to substitute the etherifying reagent for hydroxyl groups on the cellulose to form a cellulose ether. Since crystallinity has not completely disappeared in the alkali cellulose, it is industrially difficult to increase the degree of ether substitution to substitute for all hydroxyl groups on the cellulose. For this reason, commercially available cellulose ethers are water-soluble, but partially contain water-insoluble portions. Some water-insoluble portions have a scale in excess of 1,000 μm corresponding to pulp fibers as the source of cellulose.
Since water-soluble cellulose ethers create a thickened state when dissolved in water, they are often used as a thickener for clear shampoo/rinses, hair conditioners, eye drops, contact lens cleaners and the like. For example, methylcellulose and hydroxypropylcellulose, which are typical of the water-soluble cellulose ethers, exhibit surface activity owing to the inclusion of hydrophilic or hydrophobic groups within the molecule, and are used as a suspension stabilizer in suspension polymerization of vinyl chloride or vinylidene chloride, and also as a starting material to household clear wrapping film. In these applications, products are desired to be clear. Then, unless an aqueous solution of cellulose ether is in a clear state by virtue of molecular level dissolution, products will contain defective portions, leading to deficient transparency or poor function. To solve this problem, an aqueous solution of cellulose ether is filtered to remove those portions which have not been dissolved on the molecular level, prior to use. If filtration is done using a filter having an opening which is finer than necessity, the filter is frequently clogged, which obstructs the industrial manufacture.
Meanwhile, soap bubbles are often utilized as child toys, attraction events on performance stages or the like and scientific education tools because of their surface activity. To produce unbreakable bubbles, a cellulose ether may be dissolved in a bubble-forming soap solution. If the cellulose ether contains a minor amount of water-insoluble portions, these insoluble portions become defects during bubble film inflation, so that the bubble film fails. It is then difficult to blow unbreakable bubbles.